Turbines are widely used in a variety of aviation, industrial, and power generation applications to perform work. Each turbine generally includes alternating stages of peripherally mounted stator vanes and rotating blades. The stator vanes may be attached to a stationary component such as a casing that surrounds the turbine, and the rotating blades may be attached to a rotor located along an axial centerline of the turbine. A compressed working fluid, such as steam, combustion gases, or air, flows along a gas path through the turbine to produce work. The stator vanes accelerate and direct the compressed working fluid onto the subsequent stage of rotating blades to impart motion to the rotating blades, thus turning the rotor and performing work. Compressed working fluid that leaks around or bypasses the stator vanes or rotating blades reduces the efficiency of the turbine. As a result, the casing surrounding the turbine often includes a shroud or shroud segments that surround and define the outer perimeter of the gas path to reduce the amount of compressed working fluid that bypasses the stator vanes or rotating blades.
The clearance between the shroud and the rotating blades in the turbine is an important design consideration that balances efficiency and performance on the one hand with manufacturing and maintenance costs on the other hand. For example, reducing the clearance between the shroud and the rotating blades generally improves efficiency and performance of the turbine by reducing the amount of combustion gases that bypass the rotating blades. However, reduced clearances may also result in additional manufacturing costs to achieve the reduced clearances and increased maintenance costs attributed to increased rubbing, friction, or impact between the shroud and the rotating blades. The increased maintenance costs may be a particular concern in turbines in which the rotating blades rotate at speeds in excess of 1,000 revolutions per minute, have a relatively large mass, and include delicate aerodynamic surfaces. In addition, reduced clearances may result in excessive rubbing, friction, or impact between the shroud and the rotating blades during transient operations when the casing and/or shroud expands or contracts at a different rate than the rotating blades during startup, shutdown or other variations in operation.
Various systems and methods are known in the art for controlling or adjusting eccentricities between the shroud and the rotating blades. For example, U.S. Pat. No. 6,126,390 describes a passive heating-cooling system in which airflow from a compressor or combustor is metered to the turbine casing to heat or cool the turbine casing, depending on the temperature of the incoming air. U.S. patent publication 2009/0185898, assigned to the same assignee as the present invention, describes another passive system that includes an inner turbine shell having false flanges at the top and bottom to reduce eccentricities caused by transient operations.
The conventional passive systems to control or adjust eccentricities between the shroud and the rotating blades, however, assume a uniform circumferential expansion of the rotor and/or shroud and generally do not account for manufacturing or operational changes in the clearance between the shroud and the rotating blades. For example, manufacturing or assembly tolerances may produce inherent manufacturing eccentricities between the inner shroud and the rotating blades, changing the clearance between the shroud and the rotating blades around the circumference of the turbine. Similarly, bearing oil lift, thermal growth of the bearing structures, vibrations, uneven thermal expansion of the turbine components, casing slippage, gravity sag, and so forth may further change the clearance between the shroud and the rotating blades around the circumference of the turbine over time.
Anticipated manufacturing eccentricities may be accounted for by designing a minimum clearance between the shroud and the rotating blades, and some anticipated operational eccentricities may be accounted for by making static adjustments to the minimum and/or maximum clearances between the shroud and rotating blades during cold assembly. However, additional systems and methods that can actively adjust the clearance between the shroud and the rotating blades based on actual operating parameters and/or sensed operating conditions would be useful.